This invention relates to apparatus for position determination and, more particularly, to an improved apparatus and method for three-dimensional digitizing of the position of one or more moveable elements.
The determination of the location of an element in a three-dimensional space can be achieved using a three-dimensional sonic digitizer. [As used herein, "element" is intended to generically include any object or any region or point on an object.] Typically, a three-dimensional sonic digitizer operates by providing one or more sound sources that are moveable in a three-dimensional space. A plurality of sound sensors (e.g. microphones), which are at known locations, receive sounds that are periodically emitted from the sound sources. By measuring the transit time of the sound traveling from a sound source to the respective sound sensors, the distance from the sound source to each sound sensor can be obtained. The three-dimensional coordinates of the sound source can then be computed using trigonometric relationships.
The U.S. Pat. No. 3,821,469 discloses a three-dimensional digitizer of the type described above. The sound sensors used in said patent are elongated cylindrically-shaped microphones which are positioned along the three orthogonal axes of a coordinate space in which the position of a sound source is to be digitized.
An improvement on the approach of U.S. Pat. No. 3,821,469 utilizes a plurality of at least three, and preferably four, so-called point microphones mounted on a plane surface. [The fourth sensor is useful in case one of the sensors is blocked, and also for providing further information to improve accuracy.] Again, one or more sound sources located in the airspace over the four sound sensors can be digitized, the travel time to the microphones and trigonometric relationships being used to determine the three-dimensional coordinates of a sound source. Equipment of this type is known in the art and is commercially available, for example, as a Model GP-8-3D, sold by Science Accessories Corp. of Stratford, CT.
In order to accurately determine the coordinates of a sound source in the last-described type of system, it is necessary to know, with reasonably high accuracy, the relative positions of the sound sensors. Also, it is helpful to have the sensors in a predetermined regular geometric configuration, preferably a square. There are problems, however, in determining and maintaining the positions of the sound sensors with respect to each other and, for example, in a perfectly square configuration. There are a number of reasons for prior difficulties in accurately locating the sound sensors. Attempts at measuring the distance between sound sensors, such as with a ruler or by manually positioning the moveable sound sources, tend to be inaccurate due to human limitations in making the measurements. Also, the physically measured distance between, say, the outer accessible surfaces of point microphones, will not necessarily provide an accurate measure of the correct distance between these sound sensors. The desire to position the sound sensors in a near-perfect square causes further difficulty in properly and efficiently positioning all four of the sound sensors.
It is among the objects of the present invention to provide solution to difficulties of the prior art with regard to positioning of the sound sensors of a three-dimensional digitizer apparatus. It is also among the objects of the present invention to provide an improved adjustable housing unit for sound sensors of a three-dimensional digitizer apparatus.